From 24ca08a83d8b420d337d29c87d672b624890b5bf Mon Sep 17 00:00:00 2001
From: vin <git@vineetk.net>
Date: Fri, 24 May 2024 11:50:08 +0530
Subject: [PATCH] add lda immediate and brk

---
 cpu.c | 60 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 60 insertions(+)

diff --git a/cpu.c b/cpu.c
index 4a16835..0843189 100644
--- a/cpu.c
+++ b/cpu.c
@@ -40,6 +40,39 @@ uint8_t addrbus[0x10000];
 /* example program taken from https://bugzmanov.github.io/nes_ebook/chapter_3_1.html */
 uint8_t program[] = { 0xa9, 0xc0, 0xaa, 0xe8, 0x00 };
 
+void brk(void)
+{
+	/* TODO: push regs.pc and regs.status to stack and load IRQ vector */
+	regs.status.brk = 1;
+	return;
+}
+
+void lda(enum addressing_mode mode)
+{
+	uint8_t val;
+
+	switch (mode) {
+	case AM_IMMEDIATE:	/* $A9 */
+		val = program[regs.pc++];
+		regs.acc = val;
+		break;
+	case AM_ZERO_PAGE:	/* $A5 */
+	case AM_ZERO_PAGE_X:	/* $B5 */
+	case AM_ABSOLUTE:	/* $AD */
+	case AM_ABSOLUTE_X:	/* $BD */
+	case AM_ABSOLUTE_Y:	/* $B9 */
+	case AM_INDIRECT_X:	/* $A1 */
+	case AM_INDIRECT_Y:	/* $B1 */
+		/* TODO */
+		break;
+	default:
+		return;
+	}
+
+	regs.status.zero = regs.acc == 0;
+	regs.status.negative = (regs.acc & (1 << 7)) != 0;
+}
+
 void interpret(void)
 {
 	uint8_t opcode;
@@ -49,6 +82,33 @@ void interpret(void)
 		opcode = program[regs.pc++];
 
 		switch (opcode) {
+		case 0x00:
+			brk();
+			return;
+		case 0xa9:
+			lda(AM_IMMEDIATE);
+			break;
+		case 0xa5:
+			lda(AM_ZERO_PAGE);
+			break;
+		case 0xb5:
+			lda(AM_ZERO_PAGE_X);
+			break;
+		case 0xad:
+			lda(AM_ABSOLUTE);
+			break;
+		case 0xbd:
+			lda(AM_ABSOLUTE_X);
+			break;
+		case 0xb9:
+			lda(AM_ABSOLUTE_Y);
+			break;
+		case 0xa1:
+			lda(AM_INDIRECT_X);
+			break;
+		case 0xb1:
+			lda(AM_INDIRECT_Y);
+			break;
 		default:
 			break;
 		}